1. Field of the Invention
The present invention relates to an error-correcting decoder apparatus and method for correcting errors generated in digital radio communications and others.
2. Description of the Related Art
FIG. 20 of the accompanying drawings is a block diagram showing a conventional error-correcting decoder for concatenated codes disclosed in Unexamined Japanese Patent Publication No. 5-235784. In FIG. 20, an RS (Reed Solomon) encoder 101, an interleaver 102, a convolutional encoder 103, a transmission path 104, a convolutional decoder 105, a first deinterleaver 106, a comparator 107, a second deinterleaver 108, and an RS decoder 109 are shown. The RS encoder 101 adds an RS-code check symbol to the data. The interleaver 102 interleaves the transmission order of RS-encoded data. The convolutional encoder 103 convolutionally encodes data output from the interleaver 102 and the transmission path 104 transmits the data. The convolutional decoder 105 convolutionally decodes its output data and outputs data reliability information at the same time. The first deinterleaver 106 deinterleaves data output from the convolutional decoder 105. The comparator 107 compares the reliability information output from the convolutional decoder 105 with a threshold. The second deinterleaver 108 deinterleaves a signal output from the comparator 107. The RS decoder 109 RS-decodes data output from the first deinterleaver 106, based on an output signal from the second deinterleaver 108 as error-generated location information, and outputs the RS-decoded data.
Operations regarding a signal received through the transmission path 104 will be described below. Path selection is performed by the convolutional decoder 105 and a series of convolutionally decoded bits is output. At this time, the value of a path metric for the latest output data is output as reliability information. The path metric value is compared with a fixed threshold at the comparator 107. When the comparator 107 determines that the value of the reliability information is lower than the fixed threshold, the output data corresponding to the reliability information is regarded as "erasure". Then, decoded data output from the convolutional decoder 105 is deinterleaved at the first deinterleaver 106. Information on "erasure" is deinterleaved at the second deinterleaver 108 and error-correcting based on the "erasure" information is performed at the RS decoder 109. Then, a result of the error-correction is output from the RS decoder 109.
With the configuration described above, the conventional error-correcting decoder for concatenated codes has the following problem. Because the whole data is regarded as "erasure" due to convolutional decoding, the whole data is RS-decoded even when some part of the data has high reliability. Therefore, a rate of erasure symbols which are for correct parts is high. Consequently the efficiency of correction using the "erasure" is deteriorated.